Variable-shape mirror and optical pickup device therewith

ABSTRACT

The variable-shape mirror is provided with a support base and a minor portion that is disposed to face the support base and that has on a side thereof facing away from the support base a minor surface which is irradiated with a light beam. There are also provided piezoelectric elements that are sandwiched between the support base and the mirror portion and that vary the shape of the mirror surface. The piezoelectric elements are bonded, through a thin layer of metal that is separately provided for bonding, to at least one of the support base and the mirror portion. A driving voltage to the piezoelectric elements is supplied through the thin layer of metal.

This application is based on Japanese Patent Application No. 2005-179450filed on Jun. 20, 2005, the contents of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a variable-shape mirror that permitsthe shape of a mirror surface thereof to be varied, and moreparticularly relates to a bonding structure between a piezoelectricelement and other components constituting the variable-shape mirror. Thepresent invention also relates to an optical pickup device provided witha variable-shape mirror having such a bonding structure.

2. Description of Related Art

When information is read from or written to an optical disc such as a CD(compact disc) or DVD (digital versatile disc) by the use of an opticalpickup device, the relationship between the optical axis of the opticalpickup device and the disc surface should ideally be perpendicular. Inreality, however, when the disc is rotating, their relationship does notalways remain perpendicular. As a result, with an optical disc such as aCD or DVD, when its disc surface becomes inclined relative to theoptical axis, the optical path of laser light is so bent as to producecoma aberration.

When coma aberration is produced, the spot of laser light shone on theoptical disc deviates from the proper position, and, when the comaaberration becomes larger than permitted, inconveniently, it becomesimpossible to accurately write or read information. As a means tocorrect wavefront aberrations such as the coma aberration, there haveconventionally been proposed some methods for correcting aberrations bythe use of a variable-shape mirror.

For example, JP-A-H05-333274 proposes a method of performing phasecontrol by varying the shape of the mirror itself of a variable-shapemirror by the use of a plurality of actuators. Disadvantageously,however, this method is unsuitable for use in a small component such asan optical pickup device because it does not take wiring and otherfactors into consideration. Moreover, it is difficult, both technicallyand in terms of cost, to miniaturize multi-layered piezoelectricelements used as those actuators.

JP-A-2004-109562 proposes that it is advantageous in terms of lowvoltage and miniaturization to correct wavefront aberration with awavefront aberration correction mirror having a unimorph or bimorphshape provided with a piezoelectric element. It is also proposed that amirror and the piezoelectric element are bonded together with adhesivein the variable-shape mirror constructed in this way.

However, when an optical pickup device provided with a variable-shapemirror 101 that varies a mirror surface by exploiting a lateraldisplacement of a piezoelectric element as shown in FIG. 4 is used tocorrect wavefront aberration, the following problems will arise. Here,FIG. 4 is an exploded perspective view showing components constitutingthe variable-shape mirror 101.

When piezoelectric elements 104 formed of PZT (lead zirconate titanate,Pb(Zr_(x)Ti_(l-x))O₃) are bonded to both a mirror portion 103 formed ofsilicon and a support base 102 formed of glass, heat bonding will resultin an insufficient bonding strength in the bonding portions due todifferent physical properties. In this case, when a mirror surface ofthe mirror portion 103 is varied by driving the piezoelectric elements104 as shown in FIG. 5, bonding portions 105 a between the piezoelectricelements 104 and the mirror portion 103, and bonding portions 105 bbetween the piezoelectric elements 104 and the support base 102 areplaced under load. Here, FIG. 5 is a sectional view as cut along lineb-b shown in FIG. 4, showing a state in which the left-hand andright-hand piezoelectric elements 104 expand.

This increases, when bonding strength in the bonding portions isinsufficient, the possibility that the bonding between the piezoelectricelements 104 and the mirror portion 103 or the bonding between thepiezoelectric elements 104 and the support base 102 is broken when thepiezoelectric element 104 is driven, reducing the reliability of thevariable-shape mirror in terms of mechanical strength. Moreover, as theamount of expansion and contraction of the piezoelectric elements 4 isincreased, the load placed on the bonding portions 105 a and 105 b isincreased. This makes it impossible to increase the amount of expansionand contraction of the piezoelectric elements 104, thereby narrowing theaberration correction range.

For this reason, as proposed in JP-A-2004-109562, an adhesive can beused between the mirror portion 103 and the piezoelectric elements 104to increase the bonding strength between them. In this case, however,the following problem will arise. When the mirror portion 103 is formedof silicon, its electrical conductivity eliminates the need to expresslyform an electrode. However, when an adhesive is used between the mirrorportion 103 and the piezoelectric elements 104, electrical conductionbetween the mirror portion 103 and the piezoelectric elements 104 isblocked. This makes it necessary to expressly form an electrode portion.That is, the use of an adhesive complicates the construction of thevariable-shape mirror, and thus inconveniently increases the number ofcomponents.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a variable-shapemirror that enhances the bonding strength between piezoelectric elementsand a component that is subjected to the action of the piezoelectricelements with no increase in the number of components, and that properlycorrects aberrations. It is still another object of the presentinvention to provide an optical pickup device that is provided with thisvariable-shape mirror.

To achieve the above objects, according to one aspect of the presentinvention, a variable-shape mirror is provided with: a support base; amirror portion that is disposed to face the support base and that has,on a side thereof facing away from the support base, a mirror surfacewhich is irradiated with a light beam; and piezoelectric elements thatare sandwiched between the support base and the mirror portion and thatvary the shape of the mirror surface. Here, the piezoelectric elementsare bonded, by means of a thin layer of metal, to at least one of thesupport base and the mirror portion by the application of heat andpressure.

With this structure, since the strength in the bonding portions betweenthe piezoelectric elements and the support base or the mirror portion isincreased by means of a metal layer laid in between, it is possible toprovide a highly reliable variable-shape mirror without using anadhesive in the bonding portions. Moreover, since a thin layer of metalused for increasing the bonding strength has electrical conductivity,there is no need to expressly form an electrode on the mirror portion orthe support base. This helps simplify the wiring and reduce the numberof components.

In addition, the piezoelectric elements are bonded, by means of a thinlayer of metal, to projections formed in the support base by theapplication of heat and pressure. Alternatively, the piezoelectricelements are bonded, by means of a thin layer of metal, to projectionsformed in the mirror portion by the application of heat and pressure.Alternatively, the piezoelectric elements are bonded, by means of a thinlayer of metal, to projections formed in both the support base and themirror portion by the application of heat and pressure. This makes itpossible to omit the process of forming a mask on the components when athin layer of metal is formed on the projections, and thereby makes themanufacturing process easier.

Furthermore, the thin layer of metal is a thin layer of Au, and isexposed to heat and pressure at a temperature between 400° C. and 500°C. inclusive. This ensures secure bonding without using an adhesive.

According to another aspect of the present invention, an optical pickupdevice is provided with the variable-shape mirror structured asdescribed above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an exploded perspective view showing components constitutinga variable-shape mirror embodying the present invention;

FIG. 1B is a sectional view as cut along line a-a shown in FIG. 1A;

FIG. 2 is sectional view showing how the piezoelectric elements, themirror portion, and the support base of the variable-shape mirrorembodying the present invention are bonded together;

FIG. 3 is a diagram showing an outline of the optical system of anoptical pickup device employing a variable-shape mirror embodying thepresent invention;

FIG. 4 is an exploded perspective view showing components constituting aconventional variable-shape mirror; and

FIG. 5 is a sectional view as cut along line b-b shown in FIG. 4.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings. It should be understood that theembodiments described below are merely examples, and are therefore notmeant to limit in any way the manner in which the present invention canbe carried. It should also be understood that, in the drawings, thesizes and thicknesses of the components, the amount of shape variationthat occurs when the shape is varied, etc. are exaggerated for thepurpose of easy understanding, and therefore these dimensions aredifferent from those actually observed.

FIG. 1A is an exploded perspective view showing components constitutinga variable-shape mirror 1 embodying the present invention, and FIG. 1Bis a sectional view as cut along line a-a shown in FIG. 1A.

The variable-shape mirror 1 of the present invention is built as anaberration correcting mirror that varies the shape of the mirror surfaceformed on the top side of a mirror portion 3 by exploiting the verticaldisplacement of piezoelectric elements 4. The piezoelectric elements 4and fixed portions 5 are fitted to a support base 2. The support base 2is formed of, for example, an insulating material such as glass orceramic. The support base 2 has electrode holes 6 formed therein throughwhich a voltage is fed to the piezoelectric elements 4.

The mirror portion 3 reflects a light beam emitted from a light source.The mirror portion 3 is formed of, preferably, a material that is rigidand that is electrically conductive so as to be able to feed a voltageto the piezoelectric elements 4. Examples of such a material includesilicon and metals such as aluminum and iron. The mirror portion 3 maybe formed of an insulating material such as glass, though it then doesnot offer electrical conductivity. In a case where the mirror portion 3is formed of an insulating material such as glass, to achieve electricalconduction to the piezoelectric elements 4, it is necessary, forexample, to form an electrode pattern on the side of the mirror portion3 opposite to the mirror surface thereof.

The mirror portion 3 may be formed of a single material. Alternatively,it is also possible to form a base portion of the mirror portion 3 withsilicon and then coat the top side thereof by laying a coating ofaluminum or the like to form a mirror surface. It is also possible toform a plurality of layers on the base portion.

As shown in FIG. 1A, the piezoelectric elements 4 are sandwiched betweenthe support base 2 and the mirror portion 3, and four of them arearranged symmetrically on the support base 2 in cross-shaped directions.The piezoelectric element 4 expands or contracts according to thedirection of a voltage applied thereto. Used as an electrode that feedsa voltage to the piezoelectric elements 4 are, for example, a commonelectrode that is an electrically conductive mirror portion 3, andindividual electrodes patterned on the surface of the support base 2.The piezoelectric elements 4 are formed of a piezoelectric ceramic suchas PZT, for example; these, however, may be formed of any othermaterial.

In this embodiment, the piezoelectric elements 4 arerectangular-column-shaped; these are not limited to this particularshape, but may be modified within the objects of the present invention.For example, the piezoelectric elements 4 may be circular-column-shaped,or may be formed to have projections in the parts where thepiezoelectric elements 4 make contact with the mirror portion 3 or thesupport base 2. There may be provided any number of piezoelectricelements 4 in any arrangement other than specifically described in thisembodiment. However, to vary the shape of the mirror surface of themirror portion 3 uniformly at different positions, it is preferable thata plurality of piezoelectric elements 4 be provided symmetrically; whenthe size of the mirror portion 3 and other factors are taken intoconsideration, it is preferable that four piezoelectric elements 4 bearranged symmetrically in cross-shaped directions. It is still morepreferable that the piezoelectric elements 4 be arranged symmetricallyabout an axis passing through the center of the mirror surface of themirror portion 3 as seen in a plan view. In a case where a plurality ofpiezoelectric elements 4 are provided, it is preferable to adjust theheights of the piezoelectric elements 4 individually so as to preventdeformation from developing on the mirror surface of the mirror portion3.

As shown in FIGS. 1A and 1B, the fixed portions 5 are sandwiched betweenthe support base 2 and the mirror portion 3, and are disposed outsidethe piezoelectric elements 4 arranged symmetrically in cross-shapeddirections as seen in a plan view. Furthermore, the fixed portions 5are, at the top faces thereof, bonded to the mirror portion 3. In thisembodiment, the fixed portions 5 are separated from the support base 2;alternatively, the support base 2 and the fixed portions 5 may be formedintegrally, or may be given any other shapes or otherwise modifiedwithin the objects of the present invention. It is preferable that theheights of the individual fixed portions 5 be made equal to preventdeformation on the mirror surface of the mirror portion 3, and it isalso preferable that the relationship between the heights of the fixedportions 5 and the piezoelectric elements 4 be so adjusted as not toproduce deformation.

In this embodiment, as shown in FIG. 1, the variable-shape mirror 1 as awhole is given the shape of a rectangular parallelepiped; its shape,however, is not limited to this particular shape, but may be modifiedwithin the objects of the present invention. For example, the supportbase 2, the mirror portion 3, or any other components may be formedcircular, and the support base 2 may be formed larger than the mirrorportion 3.

Next, the construction according to the present invention with which thebonding strength between the piezoelectric elements 4 and the mirrorportion 3 and between the piezoelectric elements 4 and the support base2 is enhanced will be described with reference to FIG. 2. FIG. 2 is asectional view of FIG. 1B, showing a state before the piezoelectricelements 4 and the mirror portion 3 are bonded together and thepiezoelectric elements 4 and the support base 2 are bonded together.

For example, in this embodiment, a thin layer 7 of Au is formed on topfaces 4 a and bottom faces 4 b of the piezoelectric elements 4 formed ofPZT. Moreover, a thin layer 7 of Au is formed on bottom faces 3 a of themirror portion 3 formed of silicon, and is formed on top faces 2 a ofthe support base 2 formed of glass. That is, before bonding portions arebonded together, the composition of the bonding portion between themirror portion 3 and the piezoelectric elements 4 is silicon-Au-PZT; thecomposition of the bonding portion between the piezoelectric elements 4and the support base 2 is PZT-Au-glass.

The thin layer 7 of Au may be formed in any manner. For example, thethin layer 7 of Au is formed by vapor deposition or spattering.

These components are bonded together at their bonding faces where thethin layer 7 of Au is formed. At this time, these components are bondedtogether under heat and pressure, preferably, at a temperature between400° C. and 500° C. inclusive. For example, as shown in Table 1,compared to a case where these components are bonded together under heatand pressure at a temperature of 450° C., the higher the temperaturebecomes, the lower the bonding strength becomes. On the other hand, atemperature of lower than 370° C. results in insufficient bonding. Atensile strength tester (Micro Tester Model 5848 of INSTRON Corp.) wasused to measure the bonding strength.

TABLE 1 Temperature Bonding strength (° C.) (MPa) 450 22.5 500 20.4 55019.7

In this embodiment, the support base 2 is formed of glass, the mirrorportion 3 is formed of silicon, the piezoelectric elements 4 are formedof PZT, and the metal that is vapor-deposited or otherwise on thebonding faces of these components is formed of Au; these are not limitedto these particular materials, but may be modified as long as thestrength in the bonding portions can be increased, which is one objectof the present invention. That is, the metal that is vapor-deposited orotherwise onto the support base 2, the mirror portion 3, and thepiezoelectric elements 4 is not limited to Au, but may be, for example,Pt, Pd, or an alloy of, for example, Au and Pd. Moreover, the mirrorportion 3 may be formed of any material other than silicon; for example,it may be formed of Al. Furthermore, the piezoelectric elements 4 may beformed of, for example, a piezoelectric ceramic other than PZT.

Although, this embodiment deals with a case where the thin layer 7 ofmetal is formed on both bonding faces of two mutually facing componentsconstituting the bonding portions, such as the bottom of the mirrorportion 3 and the top faces 4 a of the piezoelectric elements 4, thereis no need to form the thin layer 7 of metal on both bonding faces oftwo mutually facing components. The thin layer 7 of metal may be formedon one of the bonding faces. Furthermore, in this embodiment, the thinlayer 7 of metal is not vapor-deposited or otherwise on bonding facesbetween the fixed portions 5 and the mirror portion 3. However, toenhance the bonding strength, the thin layer 7 of metal may bevapor-deposited or otherwise on the bonding faces between the fixedportions 5 and the mirror portion 3.

As described in this embodiment, to enhance the bonding strength betweenthe mirror portion 3 and the piezoelectric elements 4 and between thepiezoelectric elements 4 and the support base 2, when the thin layer 7of Au is formed on the bonding faces of these components, the highelectrical conductivity of Au eliminates the need to expressly form anelectrode to feed a voltage to the piezoelectric elements 4. That is, atthe top faces of the piezoelectric elements 4, a silicon surface of themirror portion 3 serves as a common electrode through Au, and, at thebottom faces of the piezoelectric elements 4, Au vapor-deposited on aglass surface serves as an electrode pattern, thereby making it possibleto provide individual electrodes.

In this embodiment, as shown in FIG. 1, the support base 2 and themirror portion 3 have parts 2 a and 3 a, respectively, which areprojecting parts where the support base 2 and the mirror portion 3 makecontact with the piezoelectric elements 4. It should be understood,however, this is not meant to limit in any way their shapes in practice.It is preferable, however, that one or both of the support base 2 andthe mirror portion 3 has bonding portions formed as projections. Thereason is as follows. For example, in a case where Au is vapor-depositedon the bonding faces of the support base 2 or the mirror portion 3, amask usually needs to be formed on the parts where no piezoelectricelements 4 are placed so that Au is not vapor-deposited thereon.However, by forming bonding portions as projections as in thisembodiment, it becomes possible to vapor-deposit Au without forming amask. This makes it easy to assemble the variable-shape mirror 1.

Next, as another embodiment of the present invention, an optical pickupdevice 11 employing a variable-shape mirror according to the presentinvention will be described. The optical pickup device 11 incorporatingthe variable-shape mirror 1 according to the present invention includesan optical system constructed, for example, as shown in FIG. 3. Theoptical system of the optical pickup device 11 may be constructed in anyother manner within the objects of the present invention.

The optical pickup device 11 shown in FIG. 3 is provided with asemiconductor laser 12, a collimator lens 13, a beam splitter 14, avariable-shape mirror 1 according to the present invention, aquarter-wave plate 15, an objective lens 16, a condenser lens 18, and aphotodetector 19.

The laser light emitted from the semiconductor laser 12 is convertedinto a parallel beam by the collimator lens 13. This parallel beam istransmitted through the beam splitter 14, is then reflected on thevariable-shape mirror 1, then has its polarization state changed by thequarter-wave plate 15, and is then condensed by the objective lens 16 tobe focused on an optical disc 17. The laser light reflected from theoptical disc 17 passes through the objective lens 16 and thequarter-wave plate 15, is then reflected on the variable-shape mirror 1,is then reflected by the beam splitter 14, and is then condensed by thecondenser lens 18 to be directed to the photodetector 19.

In this embodiment, the variable-shape mirror 1 functions, on one hand,as a conventionally used raising mirror. On the other hand, in thisoptical system, for example, when the optical disc 17 becomes inclinedrelative to the optical axis of the laser light, as described earlier,coma aberration is produced. To correct this coma aberration, the shapeof the mirror surface of the variable-shape mirror 1 is varied; that is,the variable-shape mirror 1 also serves to correct aberrations.Specifically, based on the signal obtained from the photodetector 19,when correction of wavefront aberrations such as coma aberration isnecessary, a controller (unillustrated) provided in the optical pickupdevice 11 feeds a signal to the variable-shape mirror 1 to instruct itto vary the shape of the mirror surface of the mirror portion 3 so as tocorrect the aberrations.

According to the present invention, a metal layer is formed between thecomponents to be bonded together so as to increase the strength in thebonding portions between the piezoelectric elements and the mirrorportion and between the piezoelectric elements and the support base.This makes it possible to provide a variable-shape mirror that offershigh reliability in terms of mechanical strength.

Since a thin layer of metal used for increasing the bonding strength haselectrical conductivity, there is no need to expressly form an electrodeon the mirror portion and the support base to feed a voltage to thepiezoelectric elements. This helps simplify the wiring and reduce thenumber of components.

Bonding portions of one or both of the mirror portion and the supportbase are so formed as to project therefrom. This makes it possible tosimplify the process of vapor-depositing or otherwise a metal layer onthese components.

With an optical pickup device according to the present inventionemploying the variable-shape mirror described above, it is possible tooffer high bonding strength and to increase the amount of shapevariation of the mirror surface. This helps properly correctaberrations.

1. A variable-shape mirror comprising: a support base; a mirror portionthat is disposed to face the support base and that has, on a sidethereof facing away from the support base, a mirror surface which isirradiated with a light beam; piezoelectric elements that are sandwichedbetween the support base and the mirror portion and that vary a shape ofthe mirror surface; and fixed portions, each of which corresponding toone of the piezoelectric elements, sandwiched between the support baseand the mirror portion, and disposed closer to an outer edge of themirror portion than said each of the corresponding piezoelectricelements is, wherein each of the piezoelectric elements has first andsecond thin layers of metal on both ends thereof, respectively, themirror portion has first projections provided on a side facing thesupport base and third thin layers of metal, each of which is providedon a bottom face of each of the first projections, and each of the thirdthin layers of metal making contact with the first thin layer of metal,the support base has individual electrodes, each of which electrodescomprises a second projection provided on the support base and a fourththin layer of metal provided on a top face of the second projection, thefourth thin layer of metal making contact with the second thin layer ofmetal, each of the fixed portions is bonded to the mirror portionwithout interposing a thin layer of metal therebetween, each of thepiezoelectric elements is bonded to the support base through the secondand fourth thin layers of metal and the second projection, and bonded tothe mirror portion through the first and third thin layers of metal andthe first projection, and when a driving voltage is applied to each ofthe piezoelectric elements through the first, second, third, and fourththin layers of metal, the piezoelectric elements expand and contractvertically to thereby deform the mirror surface.
 2. The variable-shapemirror of claim 1, wherein wherein each of the fixed portions is bondedto the support base without interposing a thin layer of metaltherebetween.
 3. The variable-shape mirror of claim 2, wherein thefirst, second, third, and fourth thin layers of metal are thin layers ofAu.
 4. The variable-shape mirror of claim 1, wherein the support baseand the fixed portions are formed integrally.
 5. The variable-shapemirror of claim 4, wherein the first, second, third, and fourth thinlayers of metal are thin layers of Au.
 6. The variable-shape mirror ofclaim 1, wherein the first, second, third. and fourth thin layers ofmetal are thin layers of Au.
 7. An optical pickup device comprising thevariable-shape mirror of claim
 1. 8. The variable-shape mirror of claim1, wherein the fixed portions are bonded only to the mirror portion andare not bonded to the support base.